IV INTRODUCTION. 



SPOROSAC. The generative sac. 



GONOTHECA or CAPSULE. The chitiiious receptacle, 

 within which the gonophores are produced, in the sub- 

 order Thecaphora. 



CORBULA. The protective case which shelters the repro- 

 ductive capsules in the genus Aglaophenia. (Woodcut, 

 fig. 37, page 308.) 



UMBRELLA or SWIMMING-BELL. The contractile disk, by 

 means of which the medusiform gonozooid propels itself. 



MANUBRIUM. The hollow body terminating in a mouth, 

 which is suspended from the top of the umbrella, and 

 hangs free in its cavity. 



VELUM. The delicate membrane which partially closes 

 the opening of the swimming-bell. 



LITHOCYSTS. Small sacs developed on the margin of 

 the swimming-bell, in certain genera, and containing 

 refractile spherules. They are probably organs of sense. 



PLANULA. The usual form of the Hydroid embryo. 



TROPHOSOME. The whole company of alimentary zooids 

 associated in a Hydroid community. 



GONOSOME. The sexual zooids of the community. 



ECTODERM. The external membrane or layer of the 

 body-substance amongst the Crelenterata. 



ENDODERM. The internal layer of the body-substance. 



NEMATOCYSTS or THREAD-CELLS. Minute sacs imbedded 

 in the body-substance containing delicate projectile threads. 



